Led by the Islamic State in Iraq and the Levant (ISIS), the insurgents are believed to be planning to push further south to the capital, Baghdad, and regions dominated by Iraq’s Shia Muslim majority, whom they regard as “infidels”.

Unconfirmed reports yesterday said Iraqi forces had launched air strikes on Mosul and Tikrit targeting the militants.

“There will be some short-term immediate things that need to be done militarily,” Mr Obama told reporters at the White House as he met Australian Prime Minister Tony Abbott.

“I don’t rule out anything because we do have a stake in making sure these jihadists are not getting a permanent foothold in Iraq, or Syria for that matter.”

Iraq’s ambassador to the US, Lukman Faily, earlier told the BBC this was the “most serious situation” faced by the country in recent years.

Government forces slowed the insurgents’ advance on Wednesday outside Samarra, a city just 110km (68 miles) north of Baghdad.

But reports have emerged of the rebels bypassing Samarra and seizing the town of Dhuluiya, 90km north-west of the capital.

Security in Baghdad has been stepped up after a video emerged of the militants threatening to march on the city.

The insurgents also control a large swathe of territory in eastern Syria, amid a campaign to set up a Sunni militant enclave straddling the border.

The UN Security Council has condemned the attacks on Mosul and Tikrit. The humanitarian situation around Mosul, where up to 500,000 people have fled, was “dire and… worsening by the moment”, it said. — BBC